GEOCHEMISTRY OF A JADEITITE-SERPENTINITE CONTACT, GUATEMALA

Jadeitite is found in some serpentinite bodies in a few subduction complexes. It crystallizes from Na-, Al-, and Si-rich fluids, and shows geochemical evidence of multiple subduction-zone fluid sources. The chemical effects of interactions between jadeitite-forming fluids and serpentinite host rocks are largely unknown, because intact serpentinite-to-jadeitite contacts are rarely preserved.

In the Sierra de las Minas, Guatemala, a 4-m-wide pit (likely a reworked Mesoamerican jade mine) exposes the contact between a mined-out jadeitite body and its host serpentinite. An apparent transition zone between albitite, which is the presumed outermost part of a jadeitite body that is now represented found only by a spoil pile, and nearby serpentinite outcrops contains four texturally distinct rock types of differing outcrop color. Seven samples from this zone, along with jadeitite from the spoil pile, serpentinite from outcrops located ~2 m from the outer edge of the pit, as well as two albite veins and a jadeitite nodule within the contact zone, were analyzed by XRF and LA-ICP-MS for major, minor and trace elements. FeO was determined by titration.

Al2O3, Na2O, MgO, FeO, Ni, Cr and Co abundances follow the field-defined “contact” between sheared albitite and foliated meta-ultramafic rocks. Each member of this group of elements changes abruptly across the contact from a value typical of Guatemalan jadeitite to one characteristic of local serpentinite. However, SiO2, Fe2O3, CaO, Zr, Hf, and Sc show other patterns, some of which may reflect mass transport across the contact. About 15 cm from the contact, U, Sr, Pb, Cs and Ce in albitite are greatly enriched in their abundances relative to all of the other samples. The LREE-enriched patterns of jadeitites and albitites resemble SIMS analyses of jadeite from the Nansibon jade mine in northwest Burma, and differ strikingly from the REE patterns of abyssal clinopyroxene. A conspicuous negative Eu-anomaly in the Guatemalan rocks likely reflects a seawater component. This contact testifies to complex, subduction-related interactions among multiple rock types (or element sources)and seawater.